the carbon content is increased the difference in temperature between thes,e points grows less and less, until in the harder steels the variations are hardly beyond the limits of experimental error. Moreover, there are several elements beside carbon, like manganese, phosphorus, etc., which influence the location of the critical point, so that with two steels of the same carbon content, but with varying manganese, the upper critical point of one may be lower than the lower critical point of the other.
The three critical points in a cooling bar are distinguished as Ar3, Ar2, Ar15 the point Ars being the one at the highest temperature and Arx at the lowest. In heating a bar the same three interruptions take place and the points are designated Ac^ Ac2, Ac3, it being understood that in each case the lowest numerals Acx and Ar.! refer to the lowest temperatures, and the highest numerals Ac3 and Ar3 to the highest temperatures, and that points bearing the same exponent like Acx and Art represent practically the same degree of temperature. In Fig. XV-A is shown a diagram which aims to represent the variations in the critical points for different steels. The data given by different experimenters vary considerably, but the heavy lines representing Arx, Ar2 and Ar3 are found by striking a sort of average from the available information. On each side of these heavy lines are shaded areas which represent the variations in the position of the critical point caused by differences in the content of manganese, phosphorus, etc. In the case of the 'soft steels the critical points are so far apart that the variations caused by these elements do not cause the maximum of one point to coincide with the minimum of the one just above, but as the content of carbon increases, the range between the highest and lowest critical points decreases, while the variations do not decrease, and as a consequence the maxima and minima run,.together so that they are indistinguishable.
The nature of the change that takes place at any one of these critical points is not known, but it is known that at each such point there is a great change in the micro-structure of the steel. It is known that the structure of the 'metal is quite different on either side of the critical points; that the forms, in which the iron and its alloyed constituents present themselves, change quite suddenly at certain definite points, and the structures found under certain well understood conditions are so characteristic that they form the basis of a; science, but it is not known whether the heat liberated or ab-: